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Pinterest "is about helping people to discover things they didn't know they wanted," he says

The fast-growing site has more than 11 million unique monthly users

You might excuse Ben Silbermann, co-founder and CEO of photo-sharing site Pinterest, if he looks a little overwhelmed these days.

Pinterest, which encourages users to collect images of their passions on digital pinboards and share them with others, has exploded in popularity in recent months. Reporters are besieging the soft-spoken 29-year-old for interviews.

And on Tuesday, Silbermann found himself blinking under stage lights at the South By Southwest Interactive conference, where more than 1,000 tech-literate folks packed a hall to hear him talk about his website -- one that few people had heard of six months ago.

"It's a really humbling feeling that all these people are using something that you helped make," he said during a Q&A with Hunch co-founder Chris Dixon. Their chat, a last-minute addition to the SXSW schedule, was one of the more talked-about events at the tech festival.

And Pinterest is responding to its newfound popularity with a few changes.

Silbermann said the site will soon roll out profile pages that have been redesigned to look "more beautiful" and to display users' influencers more prominently. An iPad app is also in the works, he said, although he declined to give a release date.

Many of Pinterest's core users have employed the site as a sort of online scrapbook to catalog their hobbies. The service has proved especially popular with women who use it to store and share crafts, recipes, wedding tips and home-decorating ideas.

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But Silbermann said that people are finding more surprising uses for the site as well. He cited the Museum of Modern Art in New York, which shares images from its collection; a woman in Spain who shares elaborate travel guides to cities she's visited; and a political parody page that displays GOP presidential candidate Mitt Romney's "yacht collection."

"It's exciting to see people using the product in ways we never expected," he said.

It's a far cry from two years ago, when Pinterest was struggling to find an audience.

A Yale grad with no engineering background, Silbermann worked for Google before launching Pinterest with some friends in late 2009. Real-time text feeds were the rage at the time, and some observers felt that an image-based pinboard was doomed to fail.

Nine months later, the site still had less than 10,000 users.

"I think I personally wrote to the first 5,000 users," said Silbermann, who also gave the site's users his cell phone number and met some of them for coffee.

"A lot of people ask, 'Why did you keep going? Why didn't you bail?'" he said. "I think the idea of telling people, 'We blew it,' was just too embarrassing."

Small but steady growth, and a belief in their product, kept his team going. As recently as last summer, Silbermann and a few programmers were running Pinterest from a small apartment. But then the site began to gain traction and tech blogs took notice.

"It was surreal," he said. Now he's fending off copycat sites and answering the inevitable tech-media questions about scale and monetization.

Silbermann, unlike some tech CEOs, sounds as though he'd rather talk about his product.

"To me, boards are a very human way of seeing the world," he said. "It [Pinterest] is about helping people to discover things they didn't know they wanted -- things that feel like they've been handpicked just for you."